A significant amount of effort has been placed on developing new methods and systems for providing energy from resources other than fossil fuels. Bio-based feedstocks are a resource that show promise as a renewable alternative source of hydrocarbons for producing fuel and chemicals.
Bio-based feedstocks including carbohydrates and “biomass” are materials derived from living or recently living biological materials. One type of biomass is cellulosic biomass. Cellulosic biomass is the most abundant source of carbohydrate in the world due to the lignocellulosic materials composing the cell walls. The ability to convert biomass to fuels, chemicals, energy and other materials is expected to strengthen the economy, minimize dependence on oil and gas resources, reduce air and water pollution, and decrease the net rate of carbon dioxide production.
Carbohydrates are the most abundant, naturally occurring biomolecules. Plant materials store carbohydrates either as sugars, starches, celluloses, lignocelluloses, hemicelluloses, and any combination thereof. In one embodiment, the carbohydrates include monosaccharides, polysaccharides or mixtures of monosaccharides and polysaccharides. As used herein, the term “monosaccharides” refers to hydroxy aldehydes or hydroxy ketones that cannot be hydrolyzed to smaller units. Examples of monosaccharides include, but are not limited to, dextrose, glucose, fructose and galactose. As used herein, the term “polysaccharides” refers to saccharides comprising two or more monosaccharide units. Examples of polysaccharides include, but are not limited to, cellulose, sucrose, maltose, cellobiose, and lactose. Carbohydrates are produced during photosynthesis, a process in which carbon dioxide is converted into organic compounds as a way to store energy. The carbohydrates are highly reactive compounds that can be easily oxidized to generate energy, carbon dioxide, and water. The presence of oxygen in the molecular structure of carbohydrates contributes to the reactivity of the compound. Water soluble carbohydrates react with hydrogen over catalyst(s) to generate polyols and sugar alcohols, either by hydrogenation, hydrogenolysis or both.
U.S. Publication Nos. 20080216391 and 20100076233 to Cortright et al. describes a process for converting carbohydrates to higher hydrocarbons by passing carbohydrates through a hydrogenation reaction followed by an Aqueous Phase Reforming (“APR”) process. The hydrogenation reaction produces polyhydric alcohols that can withstand the conditions present in the APR reaction. Further processing in an APR reaction and a condensation reaction can produce a higher hydrocarbon for use as a fuel. Currently APR is limited to feedstocks including sugars or starches, which competes with the use of these materials for food resulting in a limited supply. There is a need to directly process biomass into liquid fuels and industrial chemicals while avoiding or minimizing the production of unwanted by-products.
Efficient conversion of carbohydrates to glycols such as ethylene glycol or propylene glycol at high yield for use as chemical products or intermediates, has been limited by the further reaction of glycols to monooxygenates and ultimately alkanes, such that high yields of glycols cannot be obtained at high conversions of the feed carbohydrates. Use of high temperatures to increase conversion in a single reaction step can also lead to heavy ends byproduct formation from carbohydrate feeds such as biomass or soluble sugars, due to reactivity of sugar intermediates to form caramelans and tars, prior to stabilization by hydrogen. Alternate use of low conversion at lower temperatures in a single reaction step requires separation and recycle of a large stream of unconverted carbohydrates, which increases processing costs for a process which targets only glycols as the principal commercial product. Low conversion to maximize glycol yields in a single reaction steps also results in the presence of unconverted feed carbohydrates including sugars and sugar alcohols, and polyoxygenated species containing more than three oxygens. These components cause excessive coke formation and deactivation of condensation-oligomerization catalyst, upon attempted processing of the monooxygenates-rich stream obtained after glycols separation, to liquid fuels.